Today’s is a guest post, brought to you by my father, engineer, physicist, computer programmer, and Lego, Mars, and animation enthusiast: Keith Enevoldsen. If you’re not caught by the title of the post, scroll down and look at the pictures… then scroll back up and read the rest of the article because it is fascinating stuff! (P.S. He has a website full of awesome science stuff too!)

A teaser for what you have to look forward to…

~ A l i c e !

The Rocker-Bogie

All the Mars rovers have six wheels and use a rocker-bogie suspension system to drive smoothly over bumpy ground. The rocker-bogies are easy to see in pictures of the rovers (see pictures below). There is one rocker-bogie assembly on each side of the rover. The rocker is the larger link that connects to the rover body (the chassis) in the middle (at the rocker pivot), has a wheel on the front, and connects to the bogie in the back. The bogie is the smaller link that connects to the rocker in the middle (at the bogie pivot), and has wheels at both ends. Each of the six wheels has its own motor.

The Differential

It is not so easy to see and understand how the rocker-bogie mechanism keeps the body level. What prevents the rover body from tipping all the way forward or backward around the rocker pivots? If you build a model rover and you attach the rockers to the body with an axle or two pivot pins, the body will tip forward or backward until it hits the ground! In the real rovers the two rockers connect to each other and to the body through a mechanism called a differential. The differential is what keeps the body level. Relative to the body, when one rocker goes up, the other rocker goes down. Relative to the ground, the body angle is halfway between the angles of the two rockers. That’s cool, but how does it work? The different rovers use different mechanisms: a differential gearbox or a differential bar.

Differential Gearbox

The gearbox is inside the rover body, so you never see it. No wonder it is hard to figure out how it works! In my Lego model rover shown here, I use a simple three-gear differential. Two gears connect to the two rockers and the third (middle) gear connects to the body. If you hold the model rover body steady in midair and tilt one rocker up, the gears will turn and the other rocker will tilt down (see the animations below).

The real Mars Exploration Rovers use more complicated gearboxes with more gears but they are functionally equivalent to this simple three-gear differential.

Differential Bar

The Mars Science Laboratory (Curiosity) uses a differential bar. This is the big black bar that you see across the deck of the rover.

The middle of the bar is connected to the body with a pivot and the two ends are connected to the two rockers through some short links. If you hold the model rover body steady in midair and tilt one rocker up, one end of the bar will go back, the other end will go forward, and the other rocker will tilt down (see the animations below).

The Mars Exploration Rovers did not use a differential bar because it would interfere with the solar panels. But the Mars Science Laboratory does not have that problem because it is nuclear powered and has no solar panels.

Animations

Animations of a Rocker-Bogie with a Differential Gearbox

Mars Exploration Rover (Spirit and Opportunity)

Body held steady in midair:

Wheels on the ground:

Animations of a Rocker-Bogie with a Differential Bar

Mars Science Laboratory (Curiosity)
Body held steady in midair:

Wheels on the ground:

No Springs

The rocker-bogie mechanism has no springs. The absence of springs helps it keep all six wheels on the ground with approximately equal pressure on each wheel. This is a good thing when you are driving on sand!

I’d like to thank Emily Lakdawalla for her help. I had emailed her a photo of one of my earlier Lego rovers and said I knew something was missing from my rocker bogie mechanism, but I couldn’t figure out what. She supplied me with the key word “differential”.

The image you sent shows the rocker-bogie system, but not the differential. The link you send is for MER (Opprtunity and Spirit), Sojourner was before that. I still think that Sojourner had a gearbox — but my research was a LONG time ago.

In the picture if you look at the front of the rover there is a bar with a pivot in the centre just underneath the solar panel. This bar then extends around both sides of the rover and connects to an arm protruding from the top of the rotary joint where the rocker attaches to the body of the rover. This looks a lot like a differential arm to me but perhaps that’s not the actual flight model Sojourner…?

In the PDF I linked to, under section 3.3 it states:

“Rocker-bogie suspension [2]-[4][12][13] is the
system to rove on Martian surface in the mission
Mars Pathfinder”

which is of course the mission to which Sojourner belongs. It then states:

“A pair of lockers and the
body are connected via a differential bar in order to
stay pitching angle of the body at the middle of two
lockers”

However that text is about “Micro5” (not MER?) and, while I’ve read most of their references, I can’t find any other mention of Sojourners differential.

Alice, please correct the post in two places. First, strikeout “Mars Pathfinder (Sojourner)” from the sentence about gearboxes. Second, correct the caption on the photo of my Lego rover with a gearbox. It’s not Pathfinder (Sojourner), it’s MER (Spirit and Opportunity). It looks like Pathfinder because I removed the solar wings for this photo so you can see both the rocker-bogie and the (simplified) gearbox.

It’s possible that what looks like it could be a differential bar across the front, is actually part of the “stand up” mechanism. The Sojourner travelled flat (rocker straight) and had a one-way latching mechanism in the hub to lock each rocker into the bent position. The “transfer bar” may in fact be the method of actuating these latches, being driven away from the body by a motor driving a helical screw through the centre point.

I wouldn’t say the rocker-bogie is “balanced” on the differential bar. It is better to say the rocker-bogie-differential mechanism constrains the position of the body (chassis) relative to the wheels on the ground.

I’ll try to explain it some more with some more words, but it’s a clever three-dimensional mechanism and the best way to really understand it is to play with a working model. Keep looking at the pictures above when reading the words. Look carefully at all the bars (including the short links) and all the pivots.

Note: It may not be clear from the pictures which pivots are free-turning pivots and which are rigid connections. Most of the pivots you see are free-turning pivots. The rocker arms in both my models (and the bogie arms in one of my models) are bent in a wide upside-down V shape. These V-shaped bars are rigid. The angle of the V cannot change. In the rover with the differential bar, each rocker arm is connected to the body with a free pivot, and the differential bar is also connected with a free pivot. But the rover with differential gears, each rocker arm is connected rigidly to its corresponding differential gear, but the entire rocker-arm-and-gear pivots freely with respect to the body.

Let’s look at the rocker-bogie-differential in two different ways. First, imagine you are holding the model body steady up in the air in your hand. We’ll start from the body and differential bar and work down to the wheels. The differential bar is connected to the rover body top deck with a single free pivot. Each end of the differential bar is connected to each rocker arm through some short links (rods) with free pivots (as you can see in the pictures). When the differential bar turns on the pivot, the links cause the rocker arms move opposite directions — when the front of one rocker arm goes up, the front of the other rocker arm is forced down, and vice versa. One end of each rocker arm is connected to a bogie arm with a free pivot, and the bogies are free to tilt up or down without constraint when they are hanging in mid-air.

Now let’s look at it a second way, with the rover on the ground. We’ll start from the wheels and work up to the differential bar and body. Set the rover down on moderately bumpy ground (it won’t work if the ground is too craggy), first the left side, then the right. Gravity will force all six wheels to touch the ground. First consider the three wheels on the left side. The two left wheels on the bogie will touch the ground because the bogie is free to pivot. The third left wheel at the other end of the left rocker will also touch the ground because the rocker arm is free to pivot. On the right side, the right rocker arm is free to pivot relative to the left rocker (if you allow the body to freely find its own position), so all three wheels on the right side will likewise touch the ground. Now the angle of the two rocker arms relative to each other is determined by the bumps on the ground. Each rocker arm is attached to the body by a pivot and attached to the differential bar by the short links. Since the angle between the two rocker arms is set, this forces the differential bar to one particular position. The body of the rover is supported by the two rocker arm pivots and the differential bar pivot. If the body were connected only to the two rocker arm pivots, the body would tilt forward or backward (depending on whether the front or back were heavier) until it hit the ground. But the differential bar pivot holds the body in place and keeps it from tilting forward or backward. Furthermore, it holds the body is the ideal position where the tilt level of the body is exactly halfway in-between the tilt levels of the two rocker arms.

Regarding the differential gears:

My model uses the simplest differential gear system consisting of just three beveled gears. Two gears (#1 and #2) are attached rigidly to the two rocker arms. The third, middle gear (#3) is just a free gear attached to the body with a free-turning pivot. When you turn gear #1, this turns the middle gear #3, which turns gear #2 in the opposite direction from gear #1.

Springs cause the wheels to press the ground with unequal pressure. If one wheel is on a bump of sand, that wheel’s spring is compressed, and that wheel pushes down with more force than the other wheels. That may cause that wheel to push down deeper into the sand and get stuck. The rocker-bogie suspension without springs causes all wheels to press the ground with equal pressure, so it is less likely that any one wheel will get stuck in the sand.

Amazing. And only one-and-a-half decades after I published the same thing: .
By the way, this “amazing” “patented” suspension is just a wiffle tree, which was a subject thoroughly investigated in the 1950’s. Only the steerable wheels makes it novel… but you can’t trust a patent examiner to worry about novelty any more.